The present invention relates to an isolation method of a semiconductor device and more particularly, to a selective oxidation method of a buffer polysilicon which is an improved method for local oxidation of silicon (LOCOS).
Generally speaking, as the integration of semiconductor devices increases, the size of an individual element formed on a semiconductor substrate becomes reduced and the size of an isolation region which electrically isolates that individual element also becomes gradually reduced to a sub-micron level.
In such a highly integrated semiconductor device, when an isolation region is formed using a conventional LOCOS method for forming a semi-recessed field oxide film, a large bird's beak occurs, so that device isolation is impossible in the fine pattern. To solve this problem, a selective polysilicon oxidation (SEPOX) method is proposed.
FIGS.1 and 2 are cross-sectional views for illustrating a conventional SEPOX method.
Referring to FIG. 1, after forming a thin pad oxide film 2 on a semiconductor substrate 1 by a thermal oxidation process, a buffer polysilicon layer 3 and a silicon nitride film 4 are sequentially formed on pad oxide film 2. Then, a predetermined portion of silicon nitride film 4 is etched by a lithography process, thereby forming an opening (not shown) which defines an isolation region. Then, buffer polysilicon layer 3 and the surface portion of semiconductor substrate 1 which are exposed by the opening are selectively oxidized, thereby forming a field oxide film 5.
Referring to FIG. 2, silicon nitride 4 and buffer polysilicon layer 3 are removed, thereby completing the process for forming the isolation region.
According to the above-described SEPOX method, since oxidation stresses due to a volume expansion are imposed on buffer polysilicon layer 3 when field oxide film 5 is formed, the stress applied to substrate 1 and the resulting size of a bird's beak are reduced. However, also in such a SEPOX method, bird's beaks occur at two places when the size of the active region is reduced to sub-micron level and less. That is, a bird's beak occurs between pad oxide film 2 and semiconductor substrate 1 ("a" in FIG. 1 and hereinafter referred to as a bottom bird's beak), so that the size of the active region is reduced. Another bird's beak also occurs between silicon nitride film 4 and buffer polysilicon layer 3 ("b" in FIG. 1 and hereinafter referred to as a top bird's beak).
In the case of bottom bird's beak (a), the generation thereof is suppressed if the thickness of pad oxide film 2 is decreased and the thickness of buffer polysilicon layer 3 is increased. In the case of top bird's beak (b), the generation thereof cannot be suppressed even though the thicknesses of the above layers are increased. Also, if top bird's beak (b) occurs severely, as shown in FIG. 2, buffer polysilicon layer 3 is left between top bird's beaks (b) though silicon nitride film 4 and buffer polysilicon layer 3 are removed after field oxide film 5 is formed (refer to "P" in FIG. 2). Field oxide film 5, therefore, loses desirable properties of an isolation region.
For solving the problems of the SEPOX method as described above, a method where a remaining buffer polysilicon layer portion is oxidized after the field oxide film is formed is disclosed in U.S. Pat. No. 4,459,325. However, according to this method, the top bird's beak between the silicon nitride film and the buffer polysilicon layer still cannot be completely removed.